1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using an optical scanning device.
2. Description of the Background Art
Optical scanning devices employed in laser printers or the like emit an optical scanning beam from a light source that is deflected by a light deflector and then focused on a target surface using a scan/focus optical system having a f-theta lens or the like, to form a light beam spot on the target surface. The light beam spot is used to optically scan the target surface in a main scanning direction. The target surface may be a photoconductive face of a photoconductive object such as a photoconductor.
Tandem type image forming apparatuses having combinations of two or more such optical scanning devices and photoconductive objects are used to form color images (two-color images, multi-color images, etc.). As one configuration, an optical scanning unit to optically scan a plurality of photoconductive objects uses a single light deflector (such as rotatable multi-faceted mirror) to scan a plurality of photoconductive objects. Use of a single light deflector for a plurality of to-be-scanned faces enables the number of the light deflectors to be reduced, thereby enabling the image forming apparatuses to be made more compact.
In such a configuration, a plurality of optical scanning beams strikes one face of a rotatable multi-faceted mirror. However, making these multiple optical scanning beams parallel to each other in a sub-scanning direction, perpendicular to the main scanning direction, requires an increase in the size of the rotatable multi-faceted mirror along its rotational axis, which in turn increases manufacturing costs and hinders efforts to make image forming apparatuses more compact.
In view of such situation, JP-2003-5114-A discusses a system in which two or more optical scanning beams strike the same face of a rotatable multi-faceted mirror but at different angles of incidence in the sub-scanning direction (hereinafter “oblique system”). The advantage of such a system is that a plurality of optical scanning beams can be deflected simultaneously in the sub-scanning direction with even a narrow mirror face, and each optical path for each scanning beam, used to guide the deflected each scanning beam to a corresponding to-be-scanned face, can be easily separated from every other.
In such oblique system, because each optical scanning beam is deflected by a rotatable multi-faceted mirror in such a manner that a conical surface is swept around the rotation axis of the rotatable multi-faceted mirror, a so-called “skew of light flux” may occur at each deflected optical scanning beam when entering a scan lens, in which wavefront aberration of the optical scanning beam increases, focusing performance at a to-be-scanned face is degraded, the diameter of the light beam spot increases at peripheral portions in the main scanning direction, and resolution of the image deteriorates.
Further, the above-described deflection configuration may be more likely to cause a so-called “curving of scanning line.” The level of curving of the scanning line varies depending on the angle of incidence (which is not zero degrees) of the optical scanning beam in the sub-scanning direction. When an electrostatic latent image written by each optical scanning beam is developed as a toner color image and each toner color is superimposed on top of the preceding color, such curving of the scanning line may cause “misalignment of color images.”
The inventor of the present invention has discussed an optical scanning device for oblique system in JP-2006-72288-A, which may be effective for solving the above-mentioned drawbacks of wavefront aberration/curving of scanning line. Thus, in the such optical scanning device of JP-2006-72288-A, a scan/focus optical system having two lenses is employed, in which one lens at a rotatable multi-faceted mirror side is used in common for a plurality of optical scanning beams and another lens is disposed at a to-be-scanned face side for each of optical scanning beams. However, because such other lenses are typically long, such a configuration may impose undesirable design limitations on the practical layout of an optical path extending from a light deflector to each to-be-scanned faces.